Yevamot

the Rabbis are presumably of the opinion that a levirate bond does exist,  and yet it was stated in the final clause, 'And the same law applies where there are two levirs and one sister-in-law'!  Must it then be said that this represents an objection to a statement made by Rabbah son of R. Huna in the name of Rab? For Rabbah son of R. Huna stated in the name of Rab: A halizah of an impaired character must go the round of all the brothers!  — Rabbah son of R. Huna can answer you: Both according to the view of R. Gamaliel and that of the Rabbis no levirate bond exists,  and their difference here extends only to the question of a divorce that followed another divorce and a ma'amar that followed another ma'amar. The Master said, 'If he addressed a ma'amar to the one as well as to the other, he gives, according to R. Gamaliel, a letter of divorce to the first, submits also to her halizah, and is in consequence forbidden to marry her relatives, though the relatives of the second are permitted to him'. Now, consider! Since R. Gamaliel holds that there is no [validity in a] ma'amar that follows another ma'amar, the first [sister-in-law] should even be permitted to contract the levirate marriage!  — A preventive ordinance had to be made  against the possibility of the levir's marrying the second. R. Johanan said: R. Gamaliel, Beth Shammai, R. Simeon b. 'Azzai and R. Nehemiah are all of the opinion that a ma'amar constitutes a [fairly]  perfect kinyan: As to R. Gamaliel, there is the statement already mentioned. Beth Shammai? — For we learned: If two of three brothers were married to two sisters and the third was unmarried, and when one of the sisters' husbands died, the unmarried brother addressed to her a ma'amar and then his second brother died, Beth Shammai say: His wife  [remains] with him  while the other is exempt  as being his wife's sister. R. Simeon? — For it was taught: R. Simeon said to the Sages: If the cohabitation of the first  is a valid act,  that of the second cannot have any validity  if, however, the cohabitation of the first has no validity, then that of the second also has no validity.  Now, the cohabitation of one who is nine years of age has been given by the Rabbis  the same force as that of a ma'amar  and yet R. Simeon stated that such cohabitation  has no validity. Ben 'Azzai? — For it was taught: Ben 'Azzai stated, 'A ma'amar is valid after another ma'amar where it concerns two levirs  and one sister-in-law,  but no ma'amar is valid after a ma'amar where it concerns two sisters-in-law and one levir'. R. Nehemiah? — For we learned, R. NEHEMIAH SAID: WITH COHABITATION AS WITH HALIZAH WHETHER IT TOOK PLACE AT THE BEGINNING, IN THE MIDDLE, OR AT THE END, THERE IS NO VALIDITY IN ANY ACT THAT FOLLOWS IT. Now, an invalid cohabitation has been given by the Rabbis the same force as a ma'amar,  and yet it was stated, THERE IS NO VALIDITY IN ANY ACT THAT FOLLOWS IT. HOW … IF A LEVIR ADDRESSED A MA'AMAR etc.